Feature/truncation logcdf

I think this one is ready to go.

* changed generated code to use cdf-log, ccdf-log, and log-diff-exp

* added tests in parser to make sure inputs were univariate

* rewrote doc explaining relation of truncated models to cdf functions

* tested lower, upper, and lower+upper on a simple model to make sure we're still getting the same results in terms of mean and sd estimates as in 1.3